Pneumonia is a leading cause of infectious death in the United States. Alcohol predisposes the host to bacterial infections, particularly pneumonia. Alcohol abusing patients with pneumonia frequently present with granulocytopenia, which is an indicator of increased mortality. The mechanisms by which alcohol injures the marrow granulopoietic response to lung infection remain unclear. During pulmonary infection, the production of granulocyte colony-stimulating factor (G-CSF) by infected tissue is significantly increased. G-CSF is an essential granulopoietic cytokine that stimulates myeloid progenitor cell proliferation and granulocyte production in bone marrow. Binding of G-CSF to its receptor activates the p44/42-cyclin D pathway to promote myeloid progenitor cell proliferation. G-CSF also activates signal transducer and activator of transcription 3 (STAT3)-cyclin-dependent kinase (CDK) inhibitor p27Kip1 pathway that provides a negative feedback signal to cause G1 cell cycle arrest. Our preliminary data show that alcohol suppresses granulopoietic progenitor cell proliferation in response to pneumococcal pneumonia or G-CSF stimulation. Alcohol inhibits G-CSF-induced activation of the p44/42-cyclin D pathway while it enhances STAT3- p27Kip1 negative signaling. In this project, we will investigate the cell signaling mechanisms by which alcohol impairs the granulopoietic response to bacterial pneumonia. Our hypothesis is that alcohol suppresses the granulopoietic response to pneumococcal pneumonia by impairing G-CSF signaling in myeloid progenitor cells. The proposed two specific aims are: 1) To test the hypothesis that alcohol inhibits the granulopoietic response to lung infection by inhibiting G-CSF-induced activation of the p44/42-cyclin D pathway in granulopoietic cells;2) To test the hypothesis that alcohol impairs myeloid progenitor cell proliferation in response to G-CSF by enhancing the STAT3-p27Kip1 negative feedback pathway. The direct effects of alcohol versus indirect effects of oxidative stress generated from alcohol metabolism on both branches of G-CSF signaling will also be studied. This investigation will provide novel information focused on the pathogenesis of granulocytopenia in alcohol abusers with serious infections. It may also identify potential therapeutic approaches for the effective treatment of pneumonia in these immunocompromised patients.